1. Field of the Invention
The present invention relates to an optical device, and an image forming apparatus, a method for controlling of the optical device.
2. Description of the Related Art
When a charged photosensitive element is exposed to a laser beam in the dark, an electrostatic latent image formed by elimination of the exposed portion is formed on the photosensitive element. Image formation is performed by developing the electrostatic latent image and transferring and fixing the developed image on a printing medium, such as a sheet of paper. After the image formation, static electricity is eliminated from the surface of the photosensitive element. At this time, a slight electrical potential remains on the surface of the photosensitive element as a residual potential. By repeating the charging, exposure, and static elimination with respect to the photosensitive element, the residual potential is gradually increased, and the results in degradation of the quality of an image formed. Therefore, in an image forming apparatus that performs image formation by scanning a photosensitive element with a laser beam, it is necessary to measure a residual potential of the photosensitive element, and when the measured residual potential exceeds a predetermined value, a user has to take action, such as replacement of the photosensitive element.
To measure a residual potential of a photosensitive element, a light quantity of a laser beam to be emitted to the photosensitive element needs to be set to be higher than that is in image formation. Therefore, there is already known a technology to allow for not only a light quantity range in image formation (hereinafter, “imaging light quantity range”) but also up to a light quantity for measurement of the residual potential as the quantity of the laser beam and output a laser beam of the light quantity for measurement of the residual potential.
Japanese Patent Application Laid-open No. 2003-060289 discloses a configuration in which an electric current or voltage for determining the full scale of laser diode emission current in D/A conversion is input from an external terminal, and a quantity of light emitted from the laser diode is changed by changing the full scale. According to the Japanese Patent Application Laid-open No. 2003-060289, it is possible to perform correction (shading correction) for an increase in quantity of light to be delivered to the central portion of a photosensitive element, which occurs in a laser diode writing system that performs raster scanning.
Japanese Patent Application Laid-open No. 2009-015895 discloses a configuration in which the intensity of a light, which has been branched from a light directed toward a recording medium by a branch mirror and entering a photodetector for automatic power control (APC) that detects the intensity of a light output from a light source, is continuously or gradually changed by a light-quantity adjusting element (an ND filter, a polarization element, a liquid crystal element, or the like), thereby keeping a light for recording/reproduction within a dynamic range of the single photodetector for APC. According to Japanese Patent Application Laid-open No. 2009-015895, when a light for recording/reproduction is detected by the single photodetector for APC, the light for recording/reproduction can be kept within the dynamic range of the photodetector for APC.
However, in such a conventional configuration to output a laser beam for residual potential measurement, the light quantity is controlled just by simply extending a light quantity range of light used in image formation. Here this conventional configuration is specifically explained with reference to FIG. 10.
In general, a laser beam used in measurement of a residual potential of a photosensitive element is not required to have such a precise light quantity as a laser beam in formation of an image, so there is a tendency that it is only necessary to output a laser beam of roughly a target light quantity in the measurement of the residual potential. However, conventionally, the light quantity control is uniformly performed on even a light quantity of a laser beam for measurement of a residual potential, which is a higher light quantity than a range of the quantity of light used in image formation.
Specifically, when a laser beam for detection of a residual potential is output, as illustrated in FIG. 10, there is a light quantity range B from an upper limit of a light quantity range A of a laser beam used in image formation and a light quantity C for detection of a residual potential although a laser beam of a light quantity in the light quantity range B is not supposed to be emitted. The whole light quantity range including the light quantity range A and the light quantity range B is a range of the light quantity control. Therefore, a dynamic range of the light quantity control is greater than the light quantity range A, and the resolution of detection of a light quantity in the light quantity range A used in image formation becomes lower.
Furthermore, in Japanese Patent Application Laid-open No. 2003-060289 described above, the quantity of light emitted from the laser diode is changed by changing the full scale of laser diode emission current. However, the change in the light quantity from within the range of the light quantity used in image formation to the light quantity for residual potential detection is a great change from the shading-correction light quantity described above. Therefore, a dynamic range of the electric current or voltage input from the external terminal extends, and the problem of deterioration of the light quantity control accuracy with respect to the imaging light quantity range is not resolved.
Moreover, in Japanese Patent Application Laid-open No. 2009-015895 described above, a dynamic range of the light quantity is narrowed down by means of the light-quantity adjusting element, so it is possible to suppress the deterioration of the light quantity control accuracy with respect to the imaging light quantity range. However, a laser diode used in on an image forming apparatus, such as a copier, is generally integrated with a photodiode that monitors the light quantity; in this case, it is not possible to include such a light-quantity adjusting element as used in Japanese Patent Application Laid-open No. 2009-015895. Therefore, it is difficult to apply the method in Japanese Patent Application Laid-open No. 2009-015895 to a copier or the like, and the problem of the deterioration of the light quantity control accuracy still remains.
Therefore, there is a need for an optical device capable of improving the accuracy of the imaging light quantity control in a configuration to detect a residual potential of a photosensitive element.